Industrial gas turbine combustion systems usually burn hydrocarbon fuels and produce air polluting emissions such as oxides of nitrogen (NOx) and carbon monoxide (CO). Oxidization of molecular nitrogen in the gas turbine depends upon the temperature of gas located in a combustor, as well as the residence time for reactants located in the highest temperature regions within the combustor. Thus, the amount of NOx produced by the gas turbine may be reduced or controlled by either maintaining the combustor temperature below a temperature at which NOx is produced, or by limiting the residence time of the reactant in the combustor.
One approach for controlling the temperature of the combustor involves premixing fuel and air to create a fuel-air mixture prior to combustion. This approach may include the axial staging of fuel injectors where one or more injectors are located at an upstream end of the combustor and one or more injectors are located at an axially downstream location. The upstream injectors inject a first fuel-air mixture into a first or primary combustion zone where it is ignited to produce a main flow of high energy combustion gases. A second fuel-air mixture is injected into and mixed with the main flow of high energy combustion gases via a plurality of radially oriented and circumferentially spaced fuel injectors or axially staged fuel injector assemblies positioned downstream from the primary combustion zone.
Axially staged injection increases the likelihood of complete combustion of available fuel, which in turn reduces the air polluting emissions. However, with conventional axially staged fuel injection combustion systems there are various challenges with balancing air flow to the various combustor components, air flow requirements to the head end of the combustor for the first fuel-air mixture and/or compressed air flow to the axially staged fuel injectors for the second fuel-air mixture while maintaining emissions compliance over the full range of operation of the gas turbine. Therefore, an improved gas turbine combustion system which includes axially staged fuel injection would be useful in the industry.